If you walk from France to Nigeria, at what point exactly does "white" stop and "black" begin. If you walk from Beijing to Ukraine, where does "white" start.
Calling someone "asian", "white", "black", or "hispanic", might make sense within the US, but in some places, people will have no idea what your even talking about.
I know a guy who is from Urumqi, a city in the northwestern part of China.
He speaks with a strong Chinese accent, and is totally Chinese, but he hardly looks like the people from other parts of China. He has blondish hair, and you would probably call him white.
In Xinjiang, some people look very "white", others look "asian", and many look like some mix of the two. Or even "asian" but with blond hair and green eyes.
You might say this happened because a group asian people and a group of white people mixed together, but that's not really what happened at all.
As you travel across the world, light skin gradually fades into dark skin one way, and dark skin gradually faded into light skin another way. It is an gradual shade, not "black" and "white", there wasn't population bottlenecking either. There is no perfect "border" where one race crosses into another, so "races" are impossible to define in terms of biology and genetics, because it requires randomly choosing a point of reference, to represent the baseline of a certain race. People from Xinjiang, as well as much of Russia, and Central Asia, are not "mixed race", nor are they more physically diverse than people from any other parts of the world, they are simply in-between the two points that, Americans (for example), are most familiar with. If your from the US, your probably familiar with further East Asians and Europeans, this is where American immigration comes from, as well as being large population centers. On the other hand, someone from Russia, is probably also familiar with Central Asians (people from Kazakhstan, Uzbekistan, ect.).
Its only really a combination of 6-10 individual genes that effect what race you look like. All alleles are found around the world, just in different frequencies. If one trait has a high frequency in Beijing, medium frequency in Ukraine, then it might have something like a medium-high frequency in Central Asia.